The present invention disclosed herein relates to a mixed cathode active material able to complement a power decrease phenomenon and a lithium secondary battery including the same, and more particularly, to a mixed cathode active material for a lithium secondary battery having excellent effects by being used in a series-type plug-in hybrid electric vehicle (PHEV) or electric vehicle (EV) and a lithium secondary battery including the mixed cathode active material.
Recently, lithium secondary batteries have been used in various fields including portable electronic devices such as mobile phones, personal digital assistants (PDAs), and laptop computers. In particular, in line with growing concerns about environmental issues, research into lithium secondary batteries having high energy density and discharge voltage as a power source of an electric vehicle able to replace vehicles using fossil fuels such as gasoline vehicle and diesel vehicle, one of major causes of air pollution, have been actively conducted and some of the research are in a commercialization stage. Meanwhile, in order to use a lithium secondary battery as a power source of the electric vehicle, the lithium secondary battery must maintain stable power in a usable state of charge (SOC) range along with high power.
An electric vehicle is classified as a typical electric vehicle (EV), battery electric vehicle (BEV), hybrid electric vehicle (HEV), or plug-in hybrid electric vehicle (PHEV) according to a power source thereof.
The HEV among the foregoing electric vehicles is a vehicle obtaining a driving force from the combination of typical internal combustion engine (engine) and electric battery, and has a mode, in which the driving force is mainly obtained through the engine while the battery assists insufficient power of the engine only in the case of requiring more power than that of a typical case, such as uphill driving, and SOC is recovered again through charging the battery during stop of the vehicle. That is, the engine is a primary power source in the HEV, and the battery is an auxiliary power source and is only used intermittently.
The PHEV is a vehicle obtaining a driving force from the combination of engine and battery rechargeable by being connected to an external power supply, and is broadly classified as parallel-type PHEV and series-type PHEV.
In the parallel-type PHEV, the engine and the battery are in an equivalent relationship to each other as a power source and the engine or the battery may alternatingly act as a primary power source according to the situation. That is, the parallel-type PHEV is operated in a mutually parallel mode, in which the battery makes up for insufficient power of the engine when the engine becomes a primary power source and the engine makes up for insufficient power of the battery when the battery becomes a primary power source.
However, the series-type PHEV is a vehicle basically driven only by a battery, in which an engine only acts to charge the battery. Therefore, since the series-type PHEV entirely depends on the battery rather than the engine in terms of driving of the vehicle, differing from the HEV or the parallel-type PHEV, maintaining of stable power according to battery characteristics in a usable SOC range becomes a very important factor for driving safety in comparison to other types of electric vehicles, and the same also applies to the EV.
Meanwhile, with respect to LiCoO2, a typical cathode material of a high-capacity lithium secondary battery, practical limits of an increase in energy density and power characteristics have been reached. In particular, when LiCoO2 is used in high energy density applications, oxygen in a structure of LiCoO2 is discharged along with structural degeneration in a high-temperature charged state due to its structural instability to generate an exothermic reaction with an electrolyte in a battery and thus it becomes a main cause of battery explosion. In order to improve the safety limitation of LiCoO2, uses of lithium-containing manganese oxides, such as layered crystal structure LiMnO2 and spinel crystal structure LiMn2O4, and lithium-containing nickel oxide (LiNiO2) have been considered, and a great deal of research into lithium manganese oxides (hereinafter, referred to as “Mn-rich”) expressed as the following Chemical Formula 1, in which Mn as an essential transition metal is added in an amount larger than those of other transition metals (excluding lithium) to layered structure lithium manganese oxide as a high-capacity material, has recently been conducted.xLi2MnO3.(1-x)LiMO2  [Chemical Formula 1]
where 0<x<1 and M is any one element or two or more elements selected from the group consisting of aluminum (Al), magnesium (Mg), manganese (Mn), nickel (Ni), cobalt (Co), chromium (Cr), vanadium (V), and iron (Fe).
The Mn-rich has high power in a high SOC range (50% or more SOC), but the power thereof may rapidly decrease according to an increase in resistance in a low SOC range, and thus use of the Mn-rich as a cathode material of a lithium secondary battery used in the series-type PHEV or EV may be limited.
The foregoing limitations may also be generated in the case of mixing a cathode active material having an operating voltage higher than that of the Mn-rich, and the reason for this is that the Mn-rich only acts in a low SOC range.
Such limitations must be major obstacles in using the high-capacity Mn-rich in a field, in which power characteristics are regarded as particularly important, such as an electric vehicle. In particular, differing from the HEV in which an engine is a primary power source and the parallel-type PHEV in which engine and battery act as an equivalent power source, with respect to the series-type PHEV or EV that entirely depends on a battery for driving of a vehicle, the battery may be only used in a SOC range in which more than required power is maintained. When the Mn-rich is used as a cathode active material alone, power in a low SOC range decreases such that an available SOC range becomes very narrow.
Therefore, there is an urgent need for development of a cathode material, which may widen an available SOC range through maintaining power of the Mn-rich in a low SOC range and ensure a predetermined power more than the required power of the PHEV or EV.